Method for making display panels with opaque rib structures

ABSTRACT

Methods of forming plasma addressed liquid crystal display ribs are disclosed. An exemplary method includes introducing a glass paste which includes a curable medium into cavities formed in an intaglio collector to define rib structures, which are from the collector to a surface of the substrate, and removing the curable medium from the rib structures on the substrate surface to generate pores in the rib structures. The substrate having rib structures is then dipped into a solution containing an opaque pigment which is absorbed into the rib structure. Another method includes forming a temporary mask on portions of a surface of a substrate, depositing a layer of a glass paste having a curable medium and at least one pigment blended therein over the substrate and temporary mask, micro-molding rib structures from the glass paste on the substrate surface in areas between the masked sections, removing any residual layer of paste material present on the temporary mask, removing the temporary mask from the substrate, and firing the substrate with micro-molded rib structures thereon.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application discloses subject matter related to co-pendingapplications “METHOD OF MAKING GLASS STRUCTURES FOR FLAT PANEL DISPLAYS”filed on Dec. 21, 1998, as European Patent Application No. 98403245.8and on Jan. 25, 1999 as U.S. application Ser. No. 60/117,206; and U.S.application Ser. No. 08/820,206 filed Mar. 18, 1997 now U.S. Pat. No.5,853,446; the disclosures of which are incorporated herein byreference.

FIELD OF THE INVENTION

The invention relates to the field of flat panel displays, and inparticular, to the manufacture of opaque rib structures for plasmaaddressed liquid crystal (PALC) displays.

BACKGROUND INFORMATION

Flat panel displays, e.g., liquid crystal displays, are known. Recently,the use of plasma channels to address a liquid crystal display (LCD) hasbecome known. For example, U.S. Pat. Nos. 4,896,149, 5,036,317,5,077,553, 5,272,472, 5,313,223, the disclosures of which are all herebyincorporated by reference, each disclose such a structure. This type ofdisplay technology provides an active addressing matrix suitable forhigh-line-count displays, and is a competitive alternative to the knownthin-film transistor (TFT) active matrix approach.

These plasma channel panels are also referred to herein as plasmaaddressed liquid crystal (PALC) displays. This type of plasma displaypanel is generally formed of two parallel substrates separated from eachother to form a discharge space between the substrates, which is filledwith a discharge gas, such as a mixture of helium, neon and xenon. Theinner-facing surface of each of the substrates bears a pattern of spacedparallel electrodes, with the electrodes on one substrate beingoriented, for example, in a direction orthogonal to the direction of theelectrodes on the other substrate. The electrode bearing surfaces of thesubstrates are typically covered with a dielectric layer, and red, greenand blue phosphors are separately located in discrete areas on theinternal surface of the dielectric layer on one of the two substrates.The dielectric layers are generally lead-based glass frits fired between500 and 600° C., depending on their formulation and the level ofuniformity required. The displayed picture is produced by plasmadischarges which are induced locally in the gas by applying a suitablevoltage between the electrodes of one substrate and the electrodes ofthe other substrate. Ultraviolet light emitted locally by the gasdischarge induces luminescence of the neighboring phosphors.

A PALC display relies on the highly non-linear electrical behavior of arelatively low pressure (e.g., 10 to 100 Torr) gas, e.g., He, confinedin parallel channels. A cross section of a portion of a PALC display 100is shown in FIG. 1. A pair of parallel electrodes 101A (anode) and 101C(cathode) is deposited in each channel 102 on a rear glass plate 101G,for example, forming the bottom of the channels, and a very thindielectric sheet 103, e.g., a glass micro-sheet of about 50 μmthickness, forms the top of the channels 102. A liquid crystal layer 104on top of the micro-sheet 103 is the optically active portion of thedisplay 100. A cover sheet 105, e.g., a passive glass plate of about 1.1mm, with transparent conducting electrodes, e.g., made from indium-tinoxide (ITO), running perpendicular to the plasma channels 102, lies ontop of the liquid crystal 104. Conventional polarizers 106, colorfilters 107, and back lights 108, like those found in other conventionalliquid crystal displays, are also commonly used, as illustrated.

When voltages are applied to the transparent electrodes, since there isno ground plane, the voltages are divided among the liquid crystal, themicro-sheet, the plasma channel, and any other insulators interveningbetween the transparent electrode and whatever becomes the virtualground. As a practical matter, this means that if there is no plasma inthe plasma channel, the voltage drop across the liquid crystal will benegligible, and the pixels defined by the crossings of the transparentelectrodes and the plasma channels will not switch. If, however, avoltage difference sufficient to ionize the gas is first applied betweenthe pair of electrodes in a plasma channel, a plasma forms in the plasmachannel so that it becomes conducting, and constitutes a ground plane.As a result, for pixels atop this channel, the voltages will be dividedbetween the liquid crystal and the micro-sheet only. This places asubstantial voltage across the liquid crystal and causes the pixel toswitch. Igniting a plasma in the channel causes the row above thechannel to be selected. Because the gas in the channels isnon-conducting until a well-defined threshold voltage between theelectrode pair is reached, the rows are extremely well isolated from thecolumn voltages unless selected. This high non-linearity allows largenumbers of rows to be addressed without loss of contrast.

In order to avoid luminous cross-talk between neighboring regions andimprove the contrast in such displays, opaque barrier ribs 110 aredisposed on at least one of the substrates (typically the rear one)forming electrically insulated discharge cells. The barrier ribstructure is typically periodic with a pitch of, for example, from 200μm to 400 μm, depending on the panel resolution. These ribs are, forexample, about 30-100 μm wide and 100-200 μm thick (i.e., high).

Alternatively, a closed cell design has been employed having squarecells which are about 200-400 μm on each side. The “ribs” which formthese square cells are about 30 μm to 70 μm wide and about 30 to 200 μmhigh. Plasma panels of this type are described, for example, in U.S.Pat. No. 4,853,590, as well as Japanese Patent Application Nos.J04255638 and J04075232. The networks of parallel barrier ribs mentionedabove delimit columns of pixels which can be addressed independently.The two perpendicular networks of electrodes allow ionization of the gasat the selected pixels. The ultraviolet radiation emitted by the ionizedgas causes the excitation of areas of phosphorescent products associatedwith said pixels according to the configuration of an image which is tobe displayed.

In the past, the barrier ribs have typically been made either by asilk-screening method, or by sandblasting from a deposited layer offrit. Related co-pending U.S. application Ser. No. 08/820,206,referenced above, discloses micro-molding processes for making thebarrier ribs. One disadvantage associated with these micromoldingmethods, particularly when depositing opaque rib materials, is thepossibility of depositing a thin film of opaque material on the glasssubstrate between rib structures. Conventional screen printing andphotolithography-based methods typically avoid residual film formationbetween the ribs. However, for low cost processes in which it isdesirable to limit the number of printing steps, screen printing andphotolithography-based methods are limited to producing low thicknessribs, e.g., about 20 microns. Conventional methods, however, typicallyuse solvent based materials which can cause difficulties in maintainingrib shapes, particularly high aspect ratio ribs, usually requiringadditional consolidation steps to maintain rib shape. Accordingly, aneed exists for improved methods of manufacturing opaque rib structuresfor PALC displays.

SUMMARY OF THE INVENTION

This invention provides novel methods for making opaque rib structuresfor flat panel displays that utilize micro-molding techniques but thatdo not leave a residual film of opaque material between the ribstructures. The invention also provides improved micromolding methodsthat result in improved structures and lower manufacturing costs.

According to an aspect of the invention, a method of manufacturingopaque rib structures for use in a flat panel display, such as a plasmaaddressed liquid crystal (PALC) display, includes providing a substrateand an intaglio collector having cavities formed in its surfacecomplimentary to the desired size and spacing of the barrier ribs. Ahardenable glass paste which includes a glass frit and a hardenable,seftable or curable medium (hereinafter referred to collectively as“curable”), e.g., an ultra-violet sensitive medium, is provided into thecollector cavities to define rib structures. Useful curable media shouldbe micromoldable and easily removed by burning, and include boththermoplastic and thermosetting materials. However, thermoplasticmaterials are generally preferred. The rib structures are transferredfrom the collector to a surface of the substrate while being at leastpartially hardened or cured, by exposure to ultra-violet light, forexample. The curable medium, e.g., the ultra-violet sensitive medium,may then be removed from the rib structures on the substrate surface byperforming a burn-out generating porosity in the rib structures. Thesubstrate having porous rib structures may then be dipped into asolution containing an opaque pigment which is absorbed into the ribstructures, rinsed in a suitable rinsing solution, such as water andalcohol, to remove excess pigment, and fired.

According to another aspect of the invention, an alternative method ofmanufacturing opaque rib structures for use in a flat panel display,such as a plasma addressed liquid crystal (PALC) display, includes: (a)forming a temporary mask on portions of a surface of a substrate; (b)depositing a calibrated layer, i.e., a layer having a uniform thicknesswithin a close tolerance, e.g., ±10%, of a hardenable glass pastecontaining glass frit, a curable medium, e.g., a thermoplastic orthermosetting medium, and at least one opaque pigment blended therein,over the substrate and temporary mask; (c) micro-molding rib structuresof the glass paste by application of an intaglio plate or roll to thecoated substrate surface and curing the curable medium; (d) removing anyresidual layer of opaque paste material present on the temporary maskbetween the rib structures; (e) removing the temporary mask from thesubstrate; and (f) firing the substrate with micro-molded rib structuresthereon. Useful curable media should be micromoldable and easilyremoveable by burning, and are preferably thermoplastic.

According to another aspect of the invention, a further alternativemethod of manufacturing opaque rib structures for use in a flat paneldisplay, such as a plasma addressed liquid crystal (PALC) display,includes extruding a glass paste having a curable medium onto a surfaceof a substrate in a pattern defining ribs, and firing the substrate withglass paste rib pattern thereon. As in the other embodiments, usefulcurable media should be easily removable by burning, and are preferablythermoplastic. The glass paste may be impregnated with an opaquepigment, or may be made porous, dipped and rinsed, as summarized abovewith respect to the first alternative methods.

These and other aspects of the invention will become apparent from thedetailed description set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-sectional view of a conventional PlasmaAddressed Liquid Crystal (PALC) display.

FIGS. 2A to 2G illustrate a first exemplary method of forming opaque ribstructures according to the invention.

FIGS. 3A to 3G illustrate a second exemplary method of forming opaquerib structures according to the invention.

FIGS. 4A and 4B illustrate a third exemplary method of forming opaquerib structures according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will now be described in more detail by way of examplewith reference to the embodiments shown in the accompanying figures. Itshould be kept in mind that the following described embodiments are onlypresented by way of example and should not be construed as limiting theinventive concept to any particular physical configuration.

FIG. 1 illustrates a cross-sectional view of a conventional PlasmaAddressed Liquid Crystal (PALC) display, which has been brieflydiscussed in the Background section of this disclosure. A pair ofparallel electrodes 101A (anode) and 101C (cathode) is deposited in eachchannel 102 on a rear glass plate 101G, for example, forming the bottomof the channels, and a very thin dielectric sheet 103, e.g., a glassmicro-sheet of about 50 μm thickness, forms the top of the channels 102.A liquid crystal layer 104 on top of the micro-sheet 103 is theoptically active portion of the display 100. A cover sheet 105, e.g., apassive glass plate of about 1.1 mm, with transparent conductingelectrodes, e.g., made from indium-tin oxide (ITO), runningperpendicular to the plasma channels 102, lies on top of the liquidcrystal 104. Conventional polarizers 106, color filters 107, and backlights 108, like those found in other conventional liquid crystaldisplays, are also commonly used, as illustrated. Opaque ribs 110separate the channels 102.

Three exemplary alternative techniques for making opaque ribs accordingto the invention will now be described.

A first described method is illustrated in FIGS. 2A to 2G. Step 1, thefilling step, is illustrated in FIG. 2A. Prior to step 1, a pattern formaking the rib structures is predefined as a series of recesses orcavities 200 in an intaglio collector 201, and a quantity of raw ribmaterial 202 is provided. In Step 1, the raw material 202 used to makethe ribs, i.e., a glass frit paste containing a curable organic medium,is doctor-bladed into the recesses or cavities 200 formed in a softintaglio collector 201, preferably on a cylinder, filling the pattern ofcavities defining the rib structure to be made.

In Step 2, the transfer step illustrated in FIG. 2B, the paste ribmaterial 202 in the collector 201 is transferred to a surface of asubstrate 204, e.g., a glass substrate with or without electrodesalready formed thereon, to establish ribs 205 shown in FIG. 2C. If thepaste rib material 202 is curable by ultra-violet exposure, then thetransferring is done under ultra-violet (UV) exposure to at leastpartially cure the material 202 so that is will hold its rib shape afterthe transfer. If the paste rib material 202 is made of a frit and anorganic UV-sensitive medium, the material 202 is preferably UV sensitivethroughout the complete rib 205 thickness, e.g., 200 to 400 micrometers,to be able to effect complete UV curing throughout the material 202.

In Step 3, the burnout step represented by FIG. 2D, the organic mediumin the material forming the ribs 205 is removed by heating the substrateto a temperature sufficient to “burnout” the organic material, typicallyto a temperature of from about 300° C. to about 400° C. The burnout stepcauses the material forming the ribs 205 to become porous.

In Step 4, the dipping step illustrated in FIG. 2E, the substrate 204with porous ribs 205 is subjected to a dipping into a container 206having a solution 207 containing, for example, black or other opaquecolored pigments, which fill the pores in the ribs 205, rendering themopaque and/or colored. Suitable pigments include metal oxides, finepowder colored glass particles, or mixtures thereof. Although dipping ispreferred to facilitate the entry of the pigment bearing solution 207into the pores of the ribs 205, other methods of accomplishing thiscould be used, such as spraying, as would be apparent to one skilled inthe art.

Step 4 is followed by rinsing (Step 5, illustrated in FIG. 2F) in asuitable rinsing solution 208, such as water and alcohol, to remove thepigment solution from the non-porous regions of the substrate.Thereafter, the substrate is dried and fired (Step 6 illustrated in FIG.2G) at a temperature sufficient to fuse the glass frit, e.g., from about450° C. to about 600° C.

The above described process is compatible with electrode dispensingtechniques, such as, photolithography and screen-printing. In thisregard, during the process of forming the rib structures 205 on theglass substrate 204, the substrate surface between ribs remainstransparent and virtually unaffected.

Although described above with reference to UV-sensitive, i.e., curable,media, other types of curable media could be used, such as thermoplasticor heat-curable media, within the scope and spirit of the invention. Aslong as the rib structure can be formed with suitable pores forreceiving and retaining an opaque pigment, any suitable material can beused.

A second exemplary method according to the invention will now bedescribed with reference to FIGS. 3A to 3G. Prior to step 1 of themethod, a suitable substrate 301 is prepared with electrodes 302according to any of a number of known techniques, for example, a colorfilter process as disclosed in U.S. Pat. Nos. 5,514,503; 5,701,815 and5,544,582, or photolithography.

In Step 1 illustrated in FIG. 3A, a temporary mask 303, preferablyformed of a water-soluble material, is printed on the glass substrate301 already having the electrodes 302 formed thereon. The mask isprinted in a pattern covering the electrodes 303, but leaving strips 306of the substrate surface exposed between electrode pairs 302 forattachment of barrier ribs 305 to the substrate 301. A temporarywater-soluble mask is preferred so that it can be easily removed afterformation of the barrier ribs. Suitable water-soluble mask compositionsinclude, for example, gelatin, polyvinyl alcohol, andhydroxyethylcellulose.

In Step 2, illustrated in FIG. 3B, a continuous layer of substantiallyuniform thickness 304 of a curable opaque rib material, e.g., athermoplastic or thermosetting medium with a glass frit having black oropaque colored pigments, is bladed over the substrate surface 301bearing temporary mask 303. Numerous curable media could be used withinthe spirit of the invention, including, for example, a UV-sensitivemedium, as would be apparent to one skilled in the art.

In Step 3 illustrated in FIG. 3C, this calibrated layer is micro-moldedinto ribs by pressing an intaglio roll or plate having a complimentarypattern of recesses against the coated surface as described in detail inU.S. patent application Ser. No. 08/820,206 incorporated by referenceabove.

Basically this method includes embossing a coating or layer of fritcontaining material deposited directly onto a substrate with a desiredpattern. FIG. 3G illustrates details of an embodiment of suchmicro-molding, using a UV-sensitive material 312 rather than athermoplastic material. In the embodiment illustrated in FIG. 3G,recessed pattern 316 corresponds to a desired pattern for a barrier ribstructure for a plasma addressed liquid crystal display panel. Therecessed surface 316 of intaglio roll 318 contacts the frit containingmaterial 312 and embosses it to form a free standing structurecorresponding to the recessed pattern 316. If needed or desired, thefrit containing material 312 may be heated prior to contact with roll318, to impart sufficient flowability of the frit containing material312. This may be achieved, for example via radiant heating, or via hotair being blown onto the layer prior to being contacted by the intaglioroll. The material 312 is cured, e.g., via UV light 324 simultaneouswith molding of the frit containing material 312 into ribs. In the casewhere thermoplastic material is used, hardening of the ribs aftermolding would be accomplished by cooling.

Of course, if the electrodes 302 are already formed on the substrate301, as was assumed prior to step 1 of this embodiment, appropriatemeasures should be taken to assure alignment of the micro-molded ribstructures with respect to the electrodes in the final product.Alternatively, the electrodes could be formed after the micro-molded ribstructure has been formed, as would be apparent to one skilled in theart.

As can be appreciated, the layer of material to be micro-molded has adepth selected so that, during the micro-molding process, such as isillustrated in FIG. 3G, substantially all the material disposed atlocations where a rib structure will not be formed is transferred by theprocess into an adjacent area where a rib structure will be formed, andso that an ample amount of material is available for filling the mold toform the rib structure completely, taking into account any expansion dueto heating and/or contraction due to cooling. Nonetheless, a residualfilm of rib material is often left between rib structures.

In Step 4 illustrated in FIG. 3D, this thin residual layer 304′ ofmaterial present on top of the water soluble mask 303 between the ribs,is removed with an appropriate solvent.

In Step 5 illustrated in FIG. 3E, the temporary mask 303 is removed witha suitable solvent. If the mask is formed of a water-soluble material,this can be accomplished by washing with water or other aqueoussolutions. In Step 6 illustrated in FIG. 3F, thermal firing of thestructures is performed at a temperature of from about 400° C. to about600° C. degrees C. to fuse the frit.

As with the first exemplary process, this second exemplary process isalso compatible with various electrode dispensing techniques and retainsthe transparency of the substrate between the opaque ribs.

A third exemplary method according to the invention is illustrated inFIGS. 4A and 4B. In Step 1 illustrated in FIG. 4A, extrusion of the ribmaterial, e.g., a UV sensitive material 401 with frit and black or otheropaque colored pigments, through a die 402 is made under UV exposure 403directly onto a surface of a substrate 404, e.g., glass, and thenmetallic electrodes 406 are formed, e.g., printed, on the substrate 404.In Step 2 illustrated in FIG. 4B, this assembly is fired, at atemperature suitable to burn out the organic media and fuse the frit,e.g., from about 400° C. to about 600° C., to complete the process.

As may be appreciated by those skilled in the art, extrusion machine 408with die 402 deposits the rib material 401 onto the substrate 404 in apattern of parallel ribs 405. The ribs 405 could be formed in one passacross the entire substrate 404, or by multiple passes, one for each rib405, as should be apparent.

Besides the UV-sensitive binder material described above, athermoplastic or other reactive binder could be used, the process beingmodified appropriately to substitute suitable conditions to causehardening or cure of the binder material. Further, it should be apparentthat the rib material may be of the type already having an opaquepigment disposed therein, or of the type in which the pigment is addede.g. by dipping, as described above.

The materials used for making these opaque rib structures can beobtained through blends made of a lead or non-lead base glass fritfilled with adequate amounts of opacifiers added, e.g., opaque pigments,in order to obtain the desired opacity and color. For example, oxideslike Fe₂O₃, MnO, MnO₂, Cr₂O₃ and CoO can be used to obtain a blackcolor, and oxides like TiO₂ CeO₂, or ZrO₂ can be used to obtain whitecolors. As an alternative, commercial pigments could also be used.

Experiments have demonstrated the possibility of making such black ribstructures through micro-molding techniques using a devitrifying fritcompatible with a firing temperature of 550° C., the frit being filledwith a commercially available black pigment from Cerdec S. A., forexample.

It will be apparent to one skilled in the art that the manner of makingand using the claimed invention has been adequately disclosed in theabove-written description of the preferred embodiments taken togetherwith the drawings. Further, it will be understood that the abovedescribed preferred embodiments of the present invention are susceptibleto various modifications, changes, and adaptations, and the same areintended to be comprehended within the meaning and range of equivalentsof the appended claims.

What is claimed is:
 1. A method of manufacturing an opaque rib structureon a substrate, comprising: extruding a glass paste comprising a glassfrit, a curable organic medium and an opaque pigment, onto a surface ofthe substrate in a pattern defining ribs; curing said curable medium;and firing the substrate with the glass paste rib pattern thereon. 2.The method according to claim 1, wherein the substrate comprises a glasssubstrate having electrodes already formed thereon.
 3. The methodaccording to claim 1, wherein the substrate comprises a glass substrate,and wherein the method further comprises: forming electrodes on thesubstrate prior to the firing.
 4. The method according to claim 1,wherein the glass paste comprises a non-lead base glass frit.